Methods: MRI visible fiduciary markers were created using polyimide tubing (outer diameter=1.23mm, inner diameter=1.13mm). The tube was filled with jelly and sealed using bone wax at both ends. MRI fiducials were inserted into the plastic flexi needles that were placed in the interstitial brachytherapy applicator needle holes. The applicator was then placed in a container filled with water and underwent MRI scans. T2-weighted (T2W) and proton density weighted (PDW) MR images were acquired in the sagittal orientation. The echo time (TE) and repetition time (TR) were 100ms and 5,849ms for T2W images, and 5.5ms and 4,284ms for PDW images. Both T2W and PDW had an in-plane spatial resolution of 1mm and a slice thickness of 2.5mm. The number of average was 4 for both scans. The total scan time was 4 minutes and 38 seconds for T2W, and 3 minutes and 42 seconds for PDW.

Results: Fiduciary markers in all plastic flexi needles were clearly visible in both T2W and PDW images. For sagittal images, the diameter of the fiducial was smaller than the slice thickness. Due to the partial volume effect, signal from fiduciary markers was smaller than that from the water surrounding the applicator. Therefore, the windowing level was adjusted accordingly to visualize the fiduciary markers.

Conclusion: We demonstrated the feasibility of creating MRI visible fiduciary markers that could be fit into plastic needles. These markers can be used to determine applicator needle locations for interstitial brachytherapy, making treatment planning based on MRI images possible.